The invention relates to a process for cooling a hydrocarbon-rich fraction, in particular natural gas.
For the liquefaction of hydrocarbon-rich gas fractions, in particular natural gas, inter alia processes are employed in which the work-producing expansion of gases is utilized to generate refrigeration. To increase the thermodynamic efficiency, and thereby to reduce the specific energy consumption, more than one expansion turbine can be used. A shared characteristic of what are termed “multi-expander processes” is the separate provision of peak refrigeration (lowest refrigerant temperature) solely by sensible heat of a gas stream cooled by work-producing expansion and, independently thereof, the provision of the predominant part of the total required refrigeration output at a lower temperature level by using at least one further expansion turbine. Such expander processes are disclosed, for example, by U.S. Pat. No. 5,768,912, which discloses what is termed a double-N2 expander process, and also U.S. Pat. No. 6,412,302, which describes what is termed a N2—CH4 expander process.
The expander operated at the lowest temperature level, however, in this case only contributes at about 25%, typically less than 20%, to the total refrigeration output. As result, the majority of the cooling work remains with the warm expander or expanders, if more than two expanders are used.
The object of the present invention to specify a process for cooling a hydrocarbon-rich fraction, in particular natural gas, in which the refrigeration output can be distributed more evenly when two expanders are used,—in this case, the ratio is preferably 40/60 to 60/40—in order, at a given maximum size of the expanders, to increase the capacity of the liquefaction process without using parallel expanders. In addition, the use of separate refrigeration circuits, as described in the abovementioned U.S. Pat. No. 6,412,302, is to be rejected, in order to keep the capital costs low.